Radio wave absorber

ABSTRACT

There is disclosed a thinner-layered radio wave absorber having high absorption performance for a high frequency electromagnetic wave. The radio wave absorber, even when having a magnetic layer of not more than 1 mm in thickness, achieves satisfactory absorption characteristics for the high frequency electromagnetic wave by adopting a structure that a conductor is fixedly attached to a face opposite to an electromagnetic-wave incident face of the magnetic layer of single-layered structure, and also arranging the magnetic layer to have values of a real part μ′ and an imaginary part μ″ of complex relative magnetic permeability of the magnetic layer satisfying an expression of μ″≧mμ′−n (m: real number of m&gt;0, n: real number of n≧0) outside an impedance mismatching region.

TECHNICAL FIELD

[0001] This invention relates to an impedance-matching radio waveabsorber, and more specifically, to a thin-layered radio wave absorberuseful for absorption of high frequency electromagnetic waves.

BACKGROUND ART

[0002] Recently, with a demand for higher frequency of a signal usedwith electronic equipment, a problem of useless radiation emitted fromthe electronic equipment becomes marked. Control of suppressing theuseless radiation from the electronic equipment may be made by a methodfor making a change to circuit designs, or employing anti-uselessradiation components and so on. However, use of these methods becomesmore and more difficult for reasons of a demand for shorter period ofproduct span and an increase in cost or the like. In this connection,there is a tendency toward use of a method for applying an anti-uselessradiation sheet or the like made up of a sheet-shaped composite softmagnetic material having a magnetic loss even for high frequencyelectromagnetic waves.

[0003] A wireless LAN (Local Area Network), a superhighway automaticaccounting system or like communication systems making use of highfrequency radio waves have been also recently developed. However, in aradio wave-handling equipment applied to these communication systems,any radio wave other than a target signal wave results in radiodisturbance, so that development of a radio wave absorber has beenrequired for smooth communication by absorption of the generated radiodisturbance. Electromagnetic wave in the frequency band of 2.45 GHz, forinstance, is used with various kinds of electronic equipment such as anelectronic oven, a portable information terminal, a wireless LAN and aBluetooth, and smooth communication when using these pieces ofelectronic equipment without mutual malfunctions is of importance.

[0004] While the radio wave absorber is useful to transform energy ofincident radio wave into heat for absorption, it is supposed that a lossterm ∈″ (an imaginary component of a complex relative permittivity) of arelative permittivity of the radio wave absorber and a loss term μ″ (animaginary component of a complex relative magnetic permeability) of arelative magnetic permeability are related to transformation of theenergy of incident radio wave. When incidence of a radio wave on amaterial having a loss as described above occurs, the energy of theradio wave is transformed into heat for absorption according to thefollowing expression (1).

P=½ω∈₀∈^(″) |E ²|+½ωμ₀ μ″|H ²|  (1)

[0005] In the above expression (1), P represents wave absorption energy[W/m³] per unit volume, ω is the angular frequency (2πf, f: frequency ofelectromagnetic wave) of an electromagnetic wave, ∈₀ is magneticpermeability of free space, ∈″ is an imaginary component of a complexrelative permittivity (a dielectric loss), E is electric field strengthof an electromagnetic wave applied from the outside, μ₀ is magneticpermeability of free space, μ″ is an imaginary component of a complexrelative magnetic permeability(a magnetic loss), and H is magnetic fieldstrength of the electromagnetic wave applied from the outside.

[0006] According to the above expression (1), the higher loss that amaterial has, the greater will be the radio wave absorptive power.However, in a case of a plane wave in a relatively remoteelectromagnetic field at a distance of not less than λ/6 (λ: wavelengthof an electromagnetic wave) from a wave source, if incidence of a radiowave on such a high loss material just for once is all that occurs,complete absorption of energy of the radio wave for transformation intoheat is made impossible in most cases. This is because reflection takesplace on a front face of the radio wave absorber for reason of adifference in impedance between air and the radio wave absorber.

[0007] Accordingly, in the radio wave absorber for absorption of a planewave, a back face of the radio wave absorber is backed with a conductor,and absorption of the radio wave is made by a method for controllingphase of a reflected wave in an interface between the conductor and theradio wave absorber and a reflected wave in the front face of the radiowave absorber to offset the reflected waves each other. The waveabsorber implemented by taking the above method is called animpedance-matching wave absorber. The impedance-matching radio waveabsorber normally aims at a return loss of 20 dB, which is considered tobe equivalent to a value representing 99% absorption of the energy ofthe radio wave, in most cases.

[0008] It is necessary for the impedance-matching radio wave absorberused for the high frequency band of not less than 1 GHz to have highrelative magnetic permeability and high electric resistance.Conventionally, rubber ferrite, for instance, has been heretofore widelyused as a material of the impedance-matching radio wave absorber.Otherwise, carbonyl iron, form styrol carbon or the like has been alsoin use. In the impedance-matching radio wave absorber, a matchingfrequency and a matching thickness are determined once a materialconstant is established. A thickness of about 1 cm, when using rubberferrite or the like, is required for the electromagnetic wave in thefrequency band of 2.45 GHz, resulting in use of the radio wave absorberhaving the above thickness in the conventional technique. However, withthe progress of miniaturization of electronic equipment such as theportable information terminal, for instance, there is a need for smallerthickness of the radio wave absorber to reduce the proportion of a radiowave absorber size to an equipment size. In this connection, developmentof a radio wave absorber, which meets demands for smaller thickness andlighter weight while keeping up radio wave absorption performance withthe use of a material of higher relative magnetic permeability, has beendesired.

[0009] On the other hand, a thin film material containing Co is known asa material having high relative magnetic permeability enough to coverthe high frequency band, as disclosed in Japanese Patent ApplicationLaid-open No. 10-241938, for instance. Using this thin film materialmeets both high magnetic permeability and high electric resistance in aCo—Ni—Al—O thin film or the like by adopting a granular structurecomposed of two or more kinds of fine structures such as fine magneticparticles limited in particle size to about 4 to 7 nm and grainboundaries of extremely thin ceramic film surrounding the fine magneticparticles. However, the thin film material in this case is formed in theshape of a thin film using a sputtering device, resulting in noapplication to a material of practical use as the radio wave absorber.

[0010] A multi-layered radio wave absorber including a magnetic layerconsisting of the above material is also often applied as theimpedance-matching radio wave absorber. The structure available may bethat having a dielectric layer on the front face of a magnetic layerbacked with the conductor as described above and so on, for instance. Ascompared with a single-layered radio wave absorber, the multi-layeredwave absorber has advantages of easily managing matching of a reflectedwave phase by reason that reflection is subjected to the control asimpedance of an incident face nears space impedance, whereas havingdisadvantages of increasing the production cost. For that reason, inproducing the impedance-matching radio wave absorber, there is a needfor selection of a material and a structure in consideration of theabove advantages and disadvantages, while difficulty has beenexperienced in passing decision on selection of the material and thestructure.

[0011] The present invention is provided in view of the abovecircumstances, and its object is to provide a thinner-layered radio waveabsorber, which permits exact selection of a material and a structureand achieves high absorption performance for high frequencyelectromagnetic wave.

DISCLOSURE OF THE INVENTION

[0012] According to the present invention, in order to solve the aboveproblems, there is provided, in an impedance-matching radio waveabsorber, a radio wave absorber comprising a magnetic layer having athickness of not more than 1 mm and arranged to have values of a realpart μ′ and an imaginary part μ″ of complex relative magneticpermeability satisfying the expression of μ″≧mμ′−n (m: real number ofm>0, n: real number of n≧0)outside an impedance mismatching region, anda conductor fixedly attached to a face opposite to anelectromagnetic-wave incident face of the magnetic layer.

[0013] The above radio wave absorber, even when having the magneticlayer of not more than 1 mm in thickness, achieves satisfactoryabsorption characteristics for high frequency electromagnetic wave byadopting the structure that the conductor is fixedly attached to theface opposite to the electromagnetic-wave incident face of the magneticlayer of single-layered structure, and arranging the magnetic layer tohave the values of the real part μ′ and the imaginary part μ″ of thecomplex relative magnetic permeability of the magnetic layer satisfyingthe expression of μ″≧mμ′−n (m: real number of m>0, n: real number ofn≧0) outside the impedance mismatching region. When the relativepermittivity of the magnetic layer is not more than 15, the return lossof not less than 20 dB is achieved for the electromagnetic wave in thefrequency band of 2.4 to 2.5 GHz, for instance, on the assumption that4≦m≦6 and n≦30, while the return loss of not less than 10 dB is achievedon the assumption that 1.2≦m≦1.5 and n≦10. When the relativepermittivity of the magnetic layer is not more than 50, the return lossof not less than 20 dB is also achieved on the assumption that 4≦m≦6 andn≦100, while the return loss of not less than 10 dB is achieved on theassumption that 1.2≦m≦-1.5 and n≦30. Using a magnetic material of finetextural structure limited in particle size to 1 to 100 nm in the shapeof powder, for instance, for dispersion into a polymeric material or thelike permits formation of the magnetic layer.

[0014] According to the present invention, there is also provided, in animpedance-matching radio wave absorber, a radio wave absorber, whichcomprises a radio wave absorptive layer having a thickness of not morethan 1 mm and adopting a multi-layered structure including a magneticlayer arranged to have values of a real part μ′ and an imaginary part μ″of complex relative magnetic permeability satisfying the expression ofμ″≦mμ′−n (m: real number of m>0, n: real number of n≧0), and a conductorfixedly attached to a face opposite to an electromagnetic-wave incidentface of the radio wave absorptive layer.

[0015] The above radio wave absorber, even when having the magneticlayer of not more than 1 mm in thickness, achieves satisfactoryabsorption characteristics for high frequency electromagnetic wave byadopting the structure that the conductor is fixedly attached to theface opposite to the electromagnetic-wave incident face of the radiowave absorptive layer including the magnetic layer, and also arrangingthe magnetic layer to have the values of the real part μ′ and theimaginary part μ″ of the complex magnetic permeability of the magneticlayer satisfying the expression of μ″≦mμ′″n (m: real number of m>0, n:real number of n≧0). When the relative permittivity of the magneticlayer is not more than 15, the return loss of not less than 20 dB isachieved for the electromagnetic wave in the frequency band of 2.4 to2.5 GHz, for instance, on the assumption that 4≦m≦6 and n≦30, while thereturn loss of not less than 10 dB is achieved on the assumption that1.2≦m≦1.5 and n≦10. When the relative permittivity of the magnetic layeris not more than 50, the return loss of not less than 20 dB is alsoachieved on the assumption that 4≦m≦6 and n≦100, while the return lossof not less than 10 dB is achieved on the assumption that 1.2≦m≦1.5 andn≦30. Using a magnetic material of the fine textural structure limitedin particle size to 1 to 100 nm in the shape of powder, for instance,for dispersion into a polymeric material or the like permits formationof the magnetic layer. The radio wave absorptive layer has a dielectriclayer formed by kneading of ceramics with a polymeric material, forinstance, in addition to the magnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a view showing a structure of a radio wave absorber madeup of a magnetic layer of single-layered structure;

[0017]FIG. 2 shows relative magnetic permeability required in a casewhere the thickness of the magnetic layer is made variable when usingthe single-layered radio wave absorber;

[0018]FIG. 3 shows the relative magnetic permeability μ required in acase where relative permittivity ∈ of the magnetic layer is madevariable on condition that the thickness of the magnetic layer is fixed;

[0019]FIG. 4 shows the relative magnetic permeability μ required in acase where the thickness of the magnetic layer is made variable;

[0020]FIG. 5 shows the relative magnetic permeability μ required in acase where a frequency of a target electromagnetic wave is made variableon condition that the thickness of the magnetic layer is fixed;

[0021]FIG. 6 is a view showing a structure of a radio wave absorber madeup of a radio wave absorptive layer of multi-layered structure;

[0022]FIG. 7 shows the relative magnetic permeability μ required in acase where the relative permittivity dielectric constant and thethickness of a dielectric layer are made variable when using themulti-layered wave absorber;

[0023]FIG. 8 is a graph showing requirements for selection of an optimummaterial for the relative magnetic permeability of a magnetic layer;

[0024]FIG. 9 shows a matching region sufficient to achieve absorptioncharacteristics of not less than −20 dB in a case where the relativepermittivity e of the magnetic layer is assumed to be 15 when using thesingle-layered radio wave absorber;

[0025]FIG. 10 shows a matching region sufficient to achieve absorptioncharacteristics of not less than −20 dB in a case where the relativepermittivity ∈ of the magnetic layer is assumed to be 50 when using thesingle-layered radio wave absorber;

[0026]FIG. 11 shows a matching region sufficient to achieve absorptioncharacteristics of not less than −10 dB in a case where the relativepermittivity ∈ of the magnetic layer is assumed to be 1 when using thesingle-layered radio wave absorber;

[0027]FIG. 12 shows a matching region sufficient to achieve absorptioncharacteristics of not less than −10 dB in a case where the relativepermittivity ∈ of the magnetic layer is assumed to be 15 when using thesingle-layered radio wave absorber;

[0028]FIG. 13 shows a matching region sufficient to achieve absorptioncharacteristics of not less than −10 dB in a case where the relativepermittivity ∈ of the magnetic layer is assumed to be 50 when using thesingle-layered radio wave absorber; and

[0029]FIG. 14 shows wave absorption characteristics in a firstembodiment.

BEST MODE OF EMBODYING THE INVENTION

[0030] Hereinafter will be described an embodiment of the presentinvention with reference to the accompanying drawings. FIG. 1 shows astructure of a radio wave absorber made up of a magnetic layer ofsingle-layered structure.

[0031] The radio wave absorber according to the present invention is animpedance-matching radio wave absorber having a structure that a backface of a radio wave absorptive layer made up of a magnetic layer or thelike is backed with a conductor. The radio wave absorber of the abovestructure is useful for absorption of a radio wave by controlling phaseof a reflected wave in an interface between the radio wave absorptivelayer and the conductor and a reflected wave in a front face of the waveabsorptive layer so as to offset the reflected waves each other in sucha manner of allowing space impedance to match impedance of the radiowave absorptive layer by controlling the thickness of the radio waveabsorptive layer. The radio wave absorber has an effect of absorbing aplane wave in a relatively remote electromagnetic field at a distance ofnot less than λ/6 from a wave source. As one embodiment of theimpedance-matching radio wave absorber, there is shown, in FIG. 1, aradio wave absorber 10 comprising a magnetic layer 11 of single-layeredstructure that a face opposite to a wave incident direction of themagnetic layer 11 is backed with a metal plate as a conductor 12. Thesingle-layered radio wave absorber 10 as described above has advantagesof holding down its production cost, since there is less need forproduction steps, as compared with a multi-layered radio wave absorberwhich will be described later.

[0032] In general, the impedance-matching radio wave absorber giveseffect to non-reflection in a target frequency by designing a materialconstant to satisfy the following expression (2), and also bycontrolling the thickness of the radio wave absorptive layer.$\begin{matrix}{1 = {\sqrt{\frac{\mu}{ɛ}}\tanh \quad \left( {\frac{2\pi \quad f\quad d}{c}i\quad \sqrt{ɛ\quad \mu}} \right)}} & (2)\end{matrix}$

[0033] Wherein i represents an imaginary unit, and d is the thickness ofthe radio wave absorber.

[0034] In this connection, there will be given of the search for amaterial constant enough to satisfy the above expression (2) in thefollowing description. FIG. 2 is a graph showing relative magneticpermeability required in a case where the thickness of the magneticlayer 11 is made variable when using the single-layered radio waveabsorber 10 shown in FIG. 1.

[0035] In FIG. 2, there are shown values of a real part μ′and animaginary part μ″ of complex relative magnetic permeability sufficientto achieve a return loss of 20 dB for the electromagnetic wave in afrequency band of 2.45 GHz, for instance. The electromagnetic wave inthe frequency band of 2.45 GHz is used with various kinds of electronicequipment such as an electronic oven, a portable information terminaland a wireless LAN, and is supposed to be that required for smoothcommunication when using these pieces of electronic equipment withoutmutual malfunctions. Incidentally, the relative permittivity ∈ of themagnetic layer 11 is assumed to be 1. In FIG. 2, inside of each boundarygiven in a semi-elliptical shape represents a matching region meetingthe above requirements, and there are shown the matching regionsrespectively corresponding to cases where a thickness is limited to 50μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm and more than 300 μm in adescending scale. It is ascertained from FIG. 2 that the relativemagnetic permeability required for the magnetic layer 11 to meet theamount of absorption as much as −20 dB increases with the decreasingthickness of the magnetic layer 11.

[0036] Incidentally, a description regarding a dotted line in FIG. 2will be given later.

[0037]FIG. 3 is a graph showing the relative magnetic permeability μrequired in a case where the relative permittivity ∈ of the magneticlayer 11 is made variable on condition that the thickness of themagnetic layer is fixed. In FIG. 3, there is shown a case where therelative permittivity ∈ of the magnetic layer 11 is made variable oncondition that the thickness of the magnetic layer is limited to 200 μmwhen using the single-layered radio wave absorber 10, similarly to FIG.2, in which inside of each circle given outside a mismatching region 31represents values of a real part μ′and an imaginary part μ″ of thecomplex relative magnetic permeability sufficient to achieve the returnloss of 20 dB for the electromagnetic wave in the frequency band of 2.45GHz. The circles representing the regions meeting the above requirementsare shown by solid lines, broken lines and dotted lines respectivelycorresponding to cases where ∈″=0.5, 50 and 100 on condition that thevalue of the real part ∈′ of the complex relative permittivity is variedin a range from 15 to 1000 and value of the imaginary part ∈″ thereof isfurther varied depending on each value of the real part.

[0038] As a result of measurement, inside of the mismatching region 31given in a semicircular shape in FIG. 3 is considered to be a regioninsufficient to achieve the absorption characteristics of not less than−20 dB although the relative permittivity ∈ is limited to any value.Thus, even if a material that takes the values of the real part μ′ andthe imaginary part μ″ of the complex relative magnetic permeabilityincluded in the mismatching region 31 is used, no satisfactory radiowave absorber meeting the above requirements is available. It is alsounderstood that a value of the required relative magnetic permeabilitydecreases with the increasing relative permittivity dielectric constantfor achievement of the above absorption characteristics. It isascertained from FIG. 3, for instance, that supposing that a magneticmaterial of about 300 in relative permittivity is used as the magneticlayer 11, the complex relative magnetic permeability of about μ=60-40jis all that is required to produce the thin-layered radio wave absorberof 200 μm in thickness.

[0039] From a standpoint that use of the magnetic material showing therelative magnetic permeability within the above mismatching region isbeyond a range of possibility of producing a satisfactory radio waveabsorber in the present invention, examinations on a boundary of themismatching region will be given next. FIG. 4 is a graph showing therelative magnetic permeability μ required in a case where the thicknessof the magnetic layer 11 is made variable.

[0040] In FIG. 4, there is shown how a boundary of the mismatchingregion insufficient to achieve the absorption of not less than −20 dBvaries in the case where the thickness of the magnetic layer 11 is madevariable in a range from 100 to 300 μm on condition that the frequencyof a target electromagnetic wave is limited to 2.45 GHz when using thesingle-layered radio wave absorber 10 similarly. As a result, asemi-elliptical mismatching region having a similar shape to that shownin FIG. 3 has been obtained correspondingly to each thickness of themagnetic layer 11. It is ascertained from FIG. 4 that the smaller thethickness of the magnetic layer 11 is, the greater the mismatchingregion is, resulting in a need for higher relative magneticpermeability.

[0041]FIG. 5 is a graph showing the relative magnetic permeability μrequired in a case where the frequency of target electromagnetic wave ismade variable on condition that the thickness of the magnetic layer 11is fixed. In FIG. 5, there is shown how a boundary of the mismatchingregion insufficient to achieve the absorption characteristics of notless than −20 dB varies in the case where the frequency of targetelectromagnetic wave is made variable in a range from 2 to 3 GHz oncondition that the thickness of the magnetic layer 11 is limited to 200μm when using the single-layered radio wave absorber 10 similarly. As aresult, a semi-elliptical mismatching region having a similar shape tothat shown in FIG. 3 has been obtained correspondingly to theelectromagnetic wave of each frequency. It is ascertained from FIG. 5that the lower the frequency is, the higher will be the relativemagnetic permeability required to achieve the satisfactory absorptioncharacteristics for the target electromagnetic wave.

[0042] The above results may formulate a guiding principle to the effectthat there are needs for calculation of the mismatching region as shownin FIGS. 3 and 4 and also design using a material that the relativemagnetic permeability is assumed to be a value outside the mismatchingregion in order to produce the single thin-layered radio wave absorber10 having the magnetic layer 11 of not more than 1 mm in thickness.

[0043] Examinations on a radio wave absorber having a wave absorptivelayer of multi-layered structure including the magnetic layer as oneembodiment of the thin-layered radio wave absorber having the aboveabsorption performance will be given next. FIG. 6 shows a structure of aradio wave absorber made up of a radio wave absorptive layer ofmulti-layered structure, for instance.

[0044] A radio wave absorber 20 shown in FIG. 6 has a structure that adielectric layer 21 consisting of a dielectric material and a magneticlayer 22 are layered as a radio wave absorptive layer on anelectromagnetic wave incident face, and the radio wave absorptive layeris backed with a conductor 23. The radio wave absorber 20 permits areflected wave phase to be easily matched by a reason that reflection issubjected to the control as impedance of the incident face nears spaceimpedance in a manner of providing the magnetic layer 22 of highrelative magnetic permeability on the side of the backed conductor 23while providing the dielectric layer 21 on the electromagnetic-waveincident face side. Incidentally, the multi-layered radio wave absorbermay also be of a structure having a plurality of magnetic layers ordielectric layers and so on, without being limited to the abovestructure.

[0045]FIG. 7 is a graph showing a relative magnetic permeability μrequired in a case where relative permittivity and thickness of thedielectric layer 21 are made variable when using the multi-layered radiowave absorber 20 shown in FIG. 6. In FIG. 7, there are shown, in theshape of circles, regions of the relative magnetic permeabilitysufficient to achieve the absorption of not less than 20 dB in a casewhere a real part ∈′ of complex relative permittivity and the thicknessd2 of the dielectric layer 21 are made variable on condition that thedielectric layer 21 and the magnetic layer 22 are held to be 200 μm intotal thickness, the values of the real part ∈′ and an imaginary part ∈″of the complex relative permittivity of the magnetic layer 22 arerespectively assumed to be 15 and 0.5, while the values of the real partμ′ and the imaginary part μ″ of the complex relative magneticpermeability of the dielectric layer 21 are respectively assumed to be 1and 0 and the value of the imaginary part ∈″ of the complex relativepermittivity of the dielectric layer 21 is assumed to be 0 when usingthe radio wave absorber 20. Referring to FIG. 7, a semicircular rangeshown at the bottom left-hand corner is considered to be a mismatchingregion 71 insufficient to achieve the above absorption characteristicseven though the relative magnetic permeability takes on any value withinthis semicircular range, and it is also ascertained that use of themagnetic layer 22 with variations in relative magnetic permeabilityvalue is possible on the outside of the mismatching region by varyingthe relative permittivity value and the thickness of the dielectriclayer 21.

[0046] Selecting the material fit for use on the basis of the aboveresult of examinations permits implementation of the radio wave absorberof the present invention. As described the above, there may be two waysof implementing the wave absorber of the present invention, that is, oneof single-layered structure having a radio wave absorptive layercomposed of only a magnetic layer and the other of multi-layeredstructure including a magnetic layer. The single-layered radio waveabsorber has the advantages of holding down the production cost byreasons that there is less need for production steps as compared withthe multi-layered radio wave absorber having a plurality of layersbonded together, and that control of the thickness is made easier. Inthis connection, examinations on what kind of requirements is requiredfor the optimum design of the radio wave absorber will be given next.

[0047]FIG. 8 is a graph showing most suitable requirements for selectionof the materials with regard to the relative magnetic permeability ofthe magnetic layer. In FIG. 8, there is shown a region sufficient toachieve the absorption of not less than −20 dB when a frequency of thetarget electromagnetic wave is limited to 2.45 GHz, and thickness of theradio wave absorptive layer is also limited to not more than 200 μn. Asemi-elliptical range shown at the bottom left-hand corner is consideredto be a mismatching region 81, and when the relative magneticpermeability of the magnetic layer lies within this mismatching region,neither the single-layered radio wave absorber nor the multi-layeredradio wave absorber achieves the satisfactory absorption performance.

[0048] Referring to FIG. 2, when using the single-layered radio waveabsorber, the region of the relative magnetic permeability sufficient toachieve the absorption characteristics of −20 dB decreases with theincreasing thickness of the magnetic layer. However, the above region ofthe relative magnetic permeability in the drawing exists within a regionwhere the imaginary part μ″ of the complex relative magneticpermeability takes a value greater than a line that goes up from theorigin at a certain gradient, that is, within a region satisfying theexpression of μ″≧mμ′−n (m: real number of m>0, n: real number of n ≧0).When meeting the requirements in FIG. 2, the values of m and n arerespectively assumed to be about 5 and about 0. Thus, the real part μ′and the imaginary part μ″ of the complex relative magnetic permeabilityof the magnetic layer should take values meeting the requirements givenby the expression of μ″≧mμ′−n in order that the single-layered radiowave absorber may achieve the satisfactory absorption characteristicsfor certain frequency electromagnetic wave in the high frequency band.On the other hand, referring to FIG. 7, when using the multi-layeredradio wave absorber, the relative magnetic permeability of the magneticlayer may basically take values in the substantially whole region exceptthe mismatching region 81 in a manner of controlling the relativepermittivity and the thickness of the dielectric layer.

[0049] As described the above, as shown in FIG. 8, using the complexrelative magnetic permeability of the magnetic layer may apply astraight line representing the expression of μ″≧mμ′−n (m: real number ofm>0, n: real number of n≧0) as the criterion for judgment on design inorder to permit production of the wave absorber sufficient to achievethe satisfactory absorption characteristics, in a region except themismatching region 81. Accordingly, it may be judged that the regionsatisfying the expression of μ″≧mμ′−n is advantageous in producing thesingle-layered radio wave absorber, while the other region isadvantageous in producing the multi-layered radio wave absorber. In thisconnection, verifications on the values of m and n in the aboveexpression will be given next.

[0050] Firstly, a description will be given of verifications on aguiding principle on selection of a material for the magnetic layer toachieve the absorption characteristics of −20 dB when using thesingle-layered radio wave absorber. FIG. 9 shows a matching region in acase where the relative permittivity ∈ of the magnetic layer is assumedto be 15. FIG. 10 shows a matching region in a case where the relativepermittivity ∈ is assumed to be 50. Verifications on the values of m andn to meet the absorption characteristics of −20 dB with reference toFIGS. 9 and 10 will be given next.

[0051] In FIGS. 2, 9 and 10, there are shown, every value of therelative permittivity ∈ of the magnetic layer, matching regionssufficient to achieve the absorption characteristics of −20 dB in a casewhere the thickness d of the magnetic layer is made variable oncondition that the frequency of the target electromagnetic wave islimited to 2.45 GHz. The value of the relative permittivity ∈ of themagnetic layer in FIG. 2 is assumed to be 1, that in FIG. 9 is assumedto be 15 and that in FIG. 10 is assumed to be 50. In each of FIGS. 2, 9and 10, there are shown the matching regions respectively correspondingto a cases where the thickness d of the magnetic layer is limited to 50μm, 100 μm, 200 μm, 250 μm, 300 μm and more than 300 μm, as enclosedwith substantially elliptical boundaries.

[0052] In FIGS. 9 and 10, the relative magnetic permeability ∈ requiredfor the magnetic layer to meet the amount of absorption of −20 dBincreases with the decreasing thickness d, similarly to the case of FIG.2. The matching regions appear on the left side of a straight line givenat a certain gradient in each of FIGS. 9 and 10. The straight line asdescribed above is shown by dotted lines as L1, L2 and L3 respectivelyin FIGS. 2, 9 and 10. Expressing each of the straight lines L1, L2 andL3 in terms of μ″=mμ′−n gives m=5 and n=0 from FIG. 2 when the relativepermittivity ∈ is 1, gives m=4.3 and n=25 from FIG. 9 when the relativepermittivity ∈ is 15, and gives m=5.1 and n=75 from FIG. 10 when therelative permittivity ∈ is 50.

[0053] According to the above results, the guiding principle forachievement of the return loss of not less than 20 dB for the targetelectromagnetic wave in the frequency band of 2.4 to 2.5 GHz when usingthe single-layered impedance-matching radio wave absorber may be set upon the basis of values of 4≦m≦6 and n≦30 when the relative permittivity∈ of the magnetic layer is not more than 15 on condition that therelation of the real part μ′ and the imaginary part μ″ of the complexrelative magnetic permeability of the material for the magnetic layer isgiven by the expression of μ″≧mμ′−n, while being set up on the basis ofvalues of 4≦m≦6 and n≦100 when the relative permittivity ∈ of themagnetic layer is not more than 50. As described above, with theincreased value of the relative permittivity ∈ of the magnetic layer,the value of n representing the distance from the origin graduallyincreases without a greater change of the value m representing thegradient, providing a greater matching region. Using the above guidingprinciple may set up the requirements for design of the thin-layeredradio wave absorber, which is not more than 1 mm in thickness andachieves the absorption characteristics of not less than −20 dB for theuseless radio wave in the frequency band of 2.45 GHz required for alarge number of electronic equipment such as an electronic oven, aportable information terminal and a wireless LAN.

[0054] From the standpoint that effects as the radio wave absorber areaccepted so long as not less than −10 dB is achieved, verifications onthe guiding principle on selection of the material for the magneticlayer to achieve the absorption characteristics of not less than −10 dBwill be given next. FIG. 11 shows a matching region in a case where therelative permittivity ∈ of the magnetic layer is assumed to be 1. FIG.12 shows a matching region in the case where the relative permittivity ∈is assumed to be 15. FIG. 13 shows a matching region in the case wherethe relative permitivity ∈ is assumed to be 50. A description will nowbe given of verifications on the values of m and n to meet theabsorption characteristics of −10 dB with reference to FIGS. 11, 12 and13.

[0055] Referring to FIGS. 11, 12 and 13, the relative magneticpermeability μ required for the magnetic layer to meet the amount ofabsorption of −10 dB increases with the decreasing thickness d,similarly to the case of aiming at the absorption characteristics of −20dB. The matching regions appear on the left side of a straight linegiven at a certain gradient in each of FIGS. 11, 12 and 13. The straightline as described above is shown by dotted lines as L4, L5 and L6respectively in FIGS. 11, 12 and 13. Expressing each of the straightlines L4, L5 and L6 in terms of μ″=mμ′−n gives m=1.4 and n=0 from FIG.11 when the relative permittivity ∈ is 1, gives m=1.3 and n=5 from FIG.12 when the relative permittivity ∈ is 15, and gives m=1.4 and n=25 fromFIG. 13 when the relative permittivity ∈ is 50.

[0056] According to the above results, the guiding principle forachievement of the return loss of not less than 10 dB for the targetelectromagnetic wave in the frequency band of 2.4 to 2.5 GHz when usingthe single-layered impedance-matching radio wave absorber may be set upon the basis of the values of 1.2≦m≦1.5 and n≦10 when the relativepermittivity ∈ of the magnetic layer is not more than 15 on conditionthat the relation between the real part μ′ and the imaginary part μ″ ofthe complex relative magnetic permeability of the material for themagnetic layer is given by the expression of μ″≧mμ′−n, while being setup on the basis of the values of 1.3≦m≦1.5 and n≦30 when the relativepermittivity ∈ of the magnetic layer is not more than 50. As describedabove, with the increasing value of the relative permittivity ∈ of themagnetic layer, the value of n gradually increases without a greaterchange of the value of m, providing a greater matching region. Whenthere is a need for design in case of reducing the target absorptioncharacteristics to a lower level like −10 dB, the matching region basedon the single-layered radio wave absorber increases with the decreasingvalue of m, providing a greater tolerance on design.

[0057] A description will now be given of an embodiment of the waveabsorber implemented on the basis of the above requirements for design.This embodiment aims at providing the radio wave absorber, which meetsthe absorption characteristics of −20 dB for the electromagnetic wave inthe frequency band of 2.45 GHz and also the thickness of not more than 1mm.

[0058] First of all, it is necessary for a material fit for the magneticlayer to have high relative magnetic permeability enough to cover thehigh frequency band. In general, high saturated-magnetic flux density isrequired to achieve high magnetic permeability, and in this connection,FeCo alloy is known as a material, which meets the above requirements.In the present invention, high magnetic permeability and high electricresistance were achieved by using the FeCo alloy and besides, by keepingup a nano-granular structure taking the shape of fine texture composedof fine magnetic particles limited in particle size to 1 to 100 nm andextremely thin grain boundaries based on high resistance substance suchas Al₂O₃ or like ceramics surrounding the magnetic fine particles byprecipitation or the like.

[0059] Such a FeCo metal soft magnetic material may be used to combinewith a generally available polymeric material into the shape of a sheetas the radio wave absorber. When using this means, the above magneticmaterial will be prepared in the shape of powder having thenano-granular structure. The suitable particle size is in a range from10 to 50 nm in consideration of the need for filling to powder. Thethickness of the grain boundaries is preferably limited to not more thana skin depth, which is assumed to be about 1 m, so that the grainboundaries of about 0.1 to 3 μm in thickness are required. That is, interms of aspect ratio, the powder material is assumed to be in a rangefrom 50/0.1=500 at a maximum to {fraction (3/10)}=about 0.3 at aminimum. Using three pieces of rolls to knead the above powder materialdispersed into a polymeric material at a volume filling rate of 30 to60% produces a paste-like sample, which is then leveled to apredetermined thickness by a doctor blade process into the shape of asheet. As the polymeric material, chlorinated polyethylene, rubbermaterials, ABS resins, and poly-lactic acid having biodegradation or thelike may be used, or alternatively, thermosetting resins, photo-curingresins or the like may be used for hardening. Use may be also made ofconcrete, ceramics or the like, instead of the polymeric material.

[0060] The sheet-shaped material as described above was used to producethe single-layered radio wave absorber as shown in FIG. 1, as the firstembodiment. Use of the above material was capable of producing a samplethat the real part μ′ and the imaginary part μ″ of the complex relativemagnetic permeability of the magnetic layer respectively take values of5 and 80, resulting in achievement of the satisfactory absorptioncharacteristics on condition that the thickness of the magnetic layer isreduced to a far smaller value as much as 200 μm than that in theconventional art. FIG. 14 shows the radio wave absorptioncharacteristics in the first embodiment.

[0061] As shown in FIG. 14, in the first embodiment, the absorptioncharacteristics of not less than −20 dB are achieved for a TEM wave(Transverse Electric Magnetic wave) from the front face in the frequencyband around 2.45 GHz, providing the return loss of not less than 99%.Thus, effective elimination of useless radio wave is possible in thefrequency band required for the large number of electronic equipmentsuch as the electronic oven, the portable information terminal and thewireless LAN. The same absorption characteristics are also achieved in awide frequency band of about 2 to 3 GHz in the first embodiment, as tothe absorption characteristics of not less than −10 dB, at which theeffects as the radio wave absorber are to be generally accepted.Incidentally, aluminum foil was used for a backing conductor of themagnetic layer. As the backing conductor, a carbon film, an ITO (IndiumTin Oxide) film and other various kinds of metal films may be usedwithout being limited to the aluminum foil. These films may be producedas a vacuum-evaporation film or a sputtering film, or alternatively, ametal face of a structure on which the radio wave absorber is installedmay be made equivalent to the backing conductor.

[0062] Then, the above material was used to produce the radio waveabsorber having the radio wave absorptive layer of a multi-layeredstructure having the dielectric layer and the magnetic layer as shown inFIG. 6, as the second embodiment. In the second embodiment, the radiowave absorber was produced by the steps of dispersing BaO-TiO₂ ceramicsinto a polymeric material used as a base material to form a dielectriclayer as the sheet-shaped material, then pressure-bonding the dielectriclayer with a magnetic layer consisting of the same material as that inthe first embodiment after leveling the dielectric layer and themagnetic layer to a predetermined thickness, and then backing themagnetic layer with the aluminum foil. As the backed conductor, thematerials other than the aluminum foil and the processes similar tothose in the first embodiment will be also enough. Use of the abovematerial was capable of producing a sample that the dielectric constantsof the dielectric layer and the magnetic layer respectively take valuesof 300 and 15 and the real part μ′ and the imaginary part ″ of thecomplex relative magnetic permeability respectively take values of 80and 50, resulting in achievement of the absorption characteristics ofnot less than −20 dB for the electromagnetic wave in the frequency bandof 2.45 GHz on condition that the thickness of the dielectric layer andthat of the magnetic layer are respectively limited to 30 μm and 120 μm,in total 150 μm.

[0063] In the above radio wave absorber, the satisfactory absorptioncharacteristics for the high frequency electromagnetic wave may beachieved with the thickness of not more than 1 mm, while there has beenneed for a thickness of about not less than 1 cm in the conventionalart. It is extremely effective in incorporating the above radio waveabsorber in the small-sized electronic equipment such as the portableinformation terminal, for instance, since demands for smaller size andlighter weight are satisfied. The radio wave absorber produced accordingto the above method may be applied to a radio wave absorption panel anda radio wave absorption casing, without being limited to a waveabsorption sheet formed in the shape of a sheet. The method forproducing the radio wave absorber by kneading the magnetic material withthe polymeric material makes it possible to easily form the thin-layeredradio wave absorber by the steps of producing a paste-like or liquidsample in a manner of controlling the volume filling rate of themagnetic material, and then coating the face of a panel-shaped body oran electronic equipment casing with the sample by spraying or the like.

[0064] Incidentally, as the magnetic material used for the magneticlayer, either a material containing at least one of Fe, Co and Ni or analloy containing Mn such as MmAl, CnzMnAl and MnBi may be used, withoutbeing limited to the above material. As the dielectric material used forthe dielectric layer, ceramics such as PbTiO₃—PbZrO₃ (PZT ceramics),PbO₂—Li₂O₃—ZrO₂—TiO₂ (PLTZ ceramics), MgTiO₃—CaTiO₃, BaMg_(1-x)Ta_(x)O₃,BZn_(1-x)Ta_(x)O₃, Ba₂TiO₂, Zr_(1-x)Sn_(x)TiO₄, BaO—Nd₂O₃—TiO₂,Pb_(1-x)Ca_(x)ZrO₃ and PbTiO₃—PrZrO₃—PbB_(1(1-x))B_(2(x))O₃ or likeceramics may be used, without being limited to BaO—TiO₂ ceramicsdescribed the above.

INDUSTRIAL AVAILABILITY

[0065] As has been described in the foregoing, the radio wave absorberaccording to the present invention, even when having the magnetic layerof not more than 1 mm in thickness, may achieve the satisfactoryabsorption characteristics for the high frequency electromagnetic waveby adopting the structure that the conductor is fixedly attached to theface opposite to the electromagnetic-wave incident face of the magneticlayer of single-layered structure, and also by arranging the magneticlayer on the assumption that the values of the real part μ′ and theimaginary part μ″ of the complex magnetic permeability of the magneticlayer satisfy the expression of μ″≧mμ′−n (m: real number of m>0, and n:real number of n≧0) outside the impedance mismatching region.

[0066] The radio wave absorber according to the present invention, evenwhen having the magnetic layer of not more than 1 mm in thickness, mayalso achieve the satisfactory absorption characteristics for the highfrequency electromagnetic wave by adopting the structure that theconductor is fixedly attached to the face opposite to theelectromagnetic-wave incident face of the wave absorptive layerincluding the magnetic layer and also by arranging the magnetic-layer onthe assumption that the values of the real part μ′ and the imaginarypart μ″ of the complex magnetic permeability of the magnetic layersatisfy the expression of μ″≦mμ′−n (m: real number of m>0, n: realnumber of n≧0) outside the impedance mismatching region.

1. In an impedance-matching radio wave absorber, a radio wave absorbercomprising a magnetic layer having a thickness of not more than 1 mm andarranged to have values of a real part μ′ and an imaginary part μ″ ofcomplex relative magnetic permeability satisfying an expression ofμ′≧mμ′−n (m: real number of m>0, n: real number of n≧0) outside animpedance mismatching region, and a conductor fixedly attached to a faceopposite to an electromagnetic-wave incident face of said magneticlayer.
 2. A radio wave absorber according to claim 1, wherein 4≦m≦6 andn≦30 are required, when a return loss of not less than 20 dB is achievedfor said electromagnetic wave in a frequency band of 2.4 to 2.5 GHz oncondition that the relative permittivity of said magnetic layer is notmore than
 15. 3. A radio wave absorber according to claim 1, wherein4≦m≦6 and n≦100 are required, when a return loss of not less than 20 dBis achieved for said electromagnetic wave in a frequency band of 2.4 to2.5 GHz on condition that relative permittivity of said magnetic layeris not more than
 50. 4. A radio wave absorber according to claim 1,wherein 1.2≦m≦1.5 and n≦10 are required, when a return loss of not lessthan 10 dB is achieved for said electromagnetic wave in a frequency bandof 2.4 to 2.5 GHz on condition that relative permittivity of saidmagnetic layer is not more than
 15. 5. A radio wave absorber accordingto claim 1, wherein 1.3≦m≦1.5 and n≦30 are required, when the a returnloss of not less than 10 dB is achieved for said electromagnetic wave ina frequency band of 2.4 to 2.5 GHz on condition that relativepermittivity of said magnetic layer is not more than
 50. 6. A radio waveabsorber according to claim 1, wherein said magnetic layer contains amagnetic material having a fine textural structure limited in a particlesize in a range of 1 to 100 nm.
 7. A radio wave absorber according toclaim 6, wherein said magnetic material includes either a materialcontaining at least one of Fe, Co and Ni or an alloy containing Mn.
 8. Aradio wave absorber according to claim 6, wherein said magnetic layer isformed by dispersion of said magnetic material in a shape of powder intoany of a polymeric material, concrete and ceramics.
 9. In animpedance-matching radio wave absorber, a radio wave absorber comprisinga radio wave absorptive layer having a thickness of not more than 1 mmand adopting a multi-layered structure including a magnetic layerarranged to have values of a real part μ′ and an imaginary part μ″ ofcomplex relative magnetic permeability satisfying an expression ofμ″≦mμ′−n (m: real number of m>0, n: real number of n≧0) outside animpedance mismatching region, and a conductor fixedly attached to a faceopposite to an electromagnetic-wave incident face of said radio waveabsorptive layer.
 10. A radio wave absorber according to claim 9,wherein 4≦m≦6 and n≦30 are required, when a return loss of not less than20 dB is achieved for said electromagnetic wave in a frequency band of2.4 to 2.5 GHz on condition that relative permittivity of said magneticlayer is not more than
 15. 11. A radio wave absorber according to claim9, wherein 4≦m≦6 and n≦100 are required, when return loss of not lessthan 20 dB is achieved for said electromagnetic wave in a frequency bandof 2.4 to 2.5 GHz on condition that relative permittivity of saidmagnetic layer is not more than
 50. 12. A radio wave absorber accordingto claim 9, wherein 1.2≦m≦1.5 and n≦10 are required, when a return lossof not less than 10 dB is achieved for said electromagnetic wave in afrequency band of 2.4 to 2.5 GHz on condition that relative permittivityof said magnetic layer is not more than
 15. 13. A radio wave absorberaccording to claim 9, wherein 1.3≦m≦1.5 and n≦30 are required, when areturn loss of not less than 10 dB is achieved for said electromagneticwave in the frequency band of 2.4 to 2.5 GHz on condition that relativepermittivity of said magnetic layer is not more than
 50. 14. A radiowave absorber according to claim 9, wherein said magnetic layer containsa magnetic material having a fine textural structure limited in aparticle size in a range of 1 to 100 nm.
 15. A radio wave absorberaccording to claim 14, wherein said magnetic material includes either amaterial containing at least one of Fe, Co and Ni or an alloy containingMn.
 16. A radio wave absorber according to claim 14, wherein saidmagnetic material is formed by dispersion of said magnetic material in ashape of powder into any of a polymeric material, concrete and ceramics.17. A radio wave absorber according to claim 9, wherein said radio waveabsorptive layer has a dielectric layer containing a dielectricmaterial.
 18. A radio wave absorber according to claim 17, wherein saiddielectric layer is formed by kneading of said dielectric material suchas ceramics selected from a group consisting of BaO—TiO₂, PZT, PLTZ,MgTiO₃—CaTiO₃, BaMg_(1-x)Ta_(x)O₃, BZn_(1-x) Ta_(x)O₃, Ba₂TiO₂,Zr_(1-x)Sn_(x) TiO₄, BaO—Nd₂O₃—TiO₂, Pb_(1-x)Ca_(x)ZrO₃ andPbTiO₃—PrZrO₃—PbB_(1(1-x))B_(2(x))O₃ ceramics with a polymeric material.19. A radio wave absorption sheet, comprising the radio wave absorberaccording to one of claim 1 and claim
 9. 20. A radio wave absorptionpanel, comprising the radio wave absorber according to one of claim 1and claim
 9. 21. A radio wave absorption casing, comprising the radiowave absorber according to one of claim 1 and claim 9.